The present invention relates to a method for reproducing a color picture of high quality with a variable reproduction scale, for use in a picture reproducing machine of a cylinder type such as a color scanner for plate making and a color facsimile.
In a conventional color scanner of a cylinder type, some methods for varying a reproduction scale have been proposed. One of these methods is patented in Japan with the patent No. 1010876 (Patent Publication No. 52-18601), and, when this method is brought into effect, the present invention is applied. This conventional method will be described briefly in the followings.
In the conventional color scanner of a cylinder type, a picture cylinder on which an original picture is mounted, and a recording cylinder on which a photosensitive film is mounted, are synchronously rotated, and the original picture and the photosensitive film are scanned by a pickup head and a recording head which are moved in the direction of the cylinders' axes. Picture signals picked up by the pickup head are usually processed in a color separation step, a color correction step, a tone control step, and so forth, and thus the obtained recording signals are sent to the recording head and control the brightness of an exposure light source, thereby recording a reproduction picture on the photosensitive film.
In this embodiment, in order to obtain a reproduction picture having a different scale from the original picture, the ratios between the rotation speeds of the picture cylinder and the recording cylinder and the moving speeds of the pickup head and the recording head are set up to the desired values depending on the desired reproduction scale.
In this case, if the picture signals picked up by the pickup head are directly input to the recording section, since the rotation speeds of the two cylinders are different, i.e. the phases of both the sides are shifted each other, the recording position for each scanning period becomes disorder, with the result that a faithful or correct reproduction picture cannot be obtained. In order to remove this defect, the picture signals are first stored in a memory, and then they are read out of the memory in synchronization with the rotation phase of the recording cylinder in their timings, thereby recording a reproduction picture of the desired reproduction scale faithfully.
This method has been effectively practiced. However, when the reproduction scale is reduced, one scanning period of picture signals are not fed from the memory to the recording section, and hence the reproduction picture pattern is discontinuous in the skipped parts, which means the deterioration of the picture quality.
For example, when the reproduction scale is 80% or 4/5 the rotation ratio of the picture and the recording cylinders is set up to 5:4, and thus, while the picture cylinder is rotated five times, the recording cylinder is rotated four times. One scanning line of picture signals are stored in the memory every one rotation of the picture cylinder, whereas one scanning line of picture signals are read out of the memory every one rotation of the recording cylinder. Consequently, the four times of readings are carried out every five times of writings. In other words, 4/5 of the entire picture signals picked up by the pickup head are utilized for the recording.
In this embodiment, however, since the size of the reproduction picture is reduced to 80% of the original while 20% of the picture signals are skipped, the signal amount per one area unit is not changed, and, commonly, the distinguished deterioration of the picture quality is not felt. But, the defect is put in an appearance depending on kinds of the pictures.
For instance, when an oblique line of a certain angle with respect to the scanning direction exists in the picture, a pattern to be recorded in a natural shape by continuous smooth lines is recorded somewhat in a deformed shape by stepwise discontinuous lines because the parts skipped are not recorded. Particularly, since such a discontinuity occurs every certain pitches of the scanning lines, depending on reproduction scales the reproduction picture gives an unnatural impression, and the quality of the reproduction picture is conspicuously deteriorated. (For example, when the reproduction scale is 80%, the discontinuity occurs every five scanning lines.)